1. Field of the Invention
The present invention relates generally to continuous wave (CW) lasers and more specifically it relates to continuous wave lasers operating in the deep UV that do not require nonlinear optical crystal frequency convertors exposed to deep ultraviolet radiation with wavelengths less than ˜370 nm.
2. Description of Related Art
An important application carried out with the aid of laser radiation is the detection and classification of small defects inadvertently produced in the fabrication of semiconductor chips. Generally, the smaller the defect, the shorter the laser wavelength must be. Historically, this laser application was accomplished using a continuous wave (CW) argon ion gas laser emitting its radiation at a wavelength of 514 nm or 488 nm as a primary source. To achieve adequately short deep ultraviolet wavelengths of radiation for commercially viable defect detection, the 514 nm or 488 nm output radiation from an argon ion gas laser is converted to second harmonic wavelengths of 257 nm or 244 nm, respectively, by interaction with a nonlinear optical (NLO) crystal. Wavelengths shorter than ˜370 nm are designated herein as “deep ultraviolet wavelengths”, or DUV. This type of deep UV source generally provides low output power (<1 watt), is extremely inefficient (<0.01%), requires extensive electrical power conditioning and active cooling, and is physically bulky. The utilized NLO crystal degrades during use and must be refurbished frequently (see more discussion below). The stressing physical operating conditions within an argon ion gas laser generally limit its operating lifetime to <10,000 hours. Thus, there is a need to develop CW deep ultraviolet laser sources that are more than an order of magnitude more powerful and efficient (i.e., multi-watt, >1%), whose reliabilities are not compromised by NLO crystal degradation due to exposure to DUV irradiation, are much more compact, and that require only comparably benign utilities.
In an attempt to overcome some of the limitations of this above described prior art, DUV lasers based on infrared-emitting, diode-pumped solid-state lasers (DPSSLs), combined with nonlinear harmonic generation, have been developed. These lasers possess performance features that are substantially superior to argon ion gas laser based solutions. Generally, this type of laser comprises a diode-pumped solid-state gain medium (such as Nd:YAG, Nd:YVO4 or Yb fiber) emitting “fundamental” radiation in the near infrared spectral region (i.e., λIR =1064 nm), and two or more harmonic nonlinear optical (NLO) crystal converters. The NLO elements convert the fundamental IR radiation into radiation of shorter “harmonic” wavelengths: λIR/2, λIR/3, λIR/4, etc (i.e., 532 nm, 355 nm, 266 nm, respectively). Practically efficient harmonic conversion requires of the NLO crystal that: 1) its birefringence is such that the NLO conversion process is “phase-matched” (i.e., the indices of refraction at both fundamental and harmonic wavelengths are equal); 2) its nonlinearity is adequately large; 3) it is adequately transparent at all present wavelengths; and 4) its intensity threshold for optical damage substantially exceeds that of the drive intensity needed for efficient NLO conversion.
Long sustained searches for practical NLO crystals have resulted in the identification and commercial development of several NLO crystals capable of enabling practical use at output wavelengths >˜370 nm, but only a few NLO crystals possessing properties suitable for harmonic generation at wavelengths lying in the deep ultraviolet spectral region <˜370 nm. Among these latter crystals are lithium borate (LBO), beta-meta-borate (BBO), and cesium-lithium borate (CLBO).
Because the efficiency of such non-linear conversion processes scales rapidly with the intensity of the drive laser, and is degraded by poor beam quality, the drive lasers must irradiate a NLO crystal with a good spatial quality beam at intensities in the 100-1000 MW cm−2 range for efficient harmonic conversion. These intensities are readily realized by pulsed solid state lasers but are a significant challenge for CW lasers. At such intensities, nonlinear optical materials tend to degrade during operation due to optical damage. This is particularly so for NLO crystals operating with output wavelengths below ˜370 nm. Thus, 355 nm and 266 nm lasers based on the use of NLO crystals tend to degrade with operating time, so that their relatively short operating times before refurbishment or replacement becomes a cost driver for users. To be commercially viable, complicated and expensive defensive measures have been adopted, such as translating the nonlinear optical crystal transverse to the drive laser beam to operate in an undamaged region of the crystal.
For some inspection applications a purely continuous wave (i.e., not repetitively pulsed) optical laser source is required to avoid optical damage to the specimen being inspected. A prior art purely CW 266 nm source of laser radiation has been based on fourth harmonic generation of the 1064 nm fundamental radiation from a diode-pumped solid state laser [T. Suedmeyer, Optics Express, 16 (3) 1546 (2008)].
This type of laser consists of three major subsystems: 1) a CW, high-power 1064 nm Master-Oscillator-Power-Amplifier (MOPA) drive laser: 2) a second harmonic converter subsystem comprising, a first optical cavity containing a first non-linear crystal operating in the visible spectral region; and 3) a fourth harmonic converter subsystem comprising a second optical cavity and a second nonlinear crystal operating in the DUV spectral region. To respond to the need for high drive laser intensities at the nonlinear crystal to achieve practically high conversion efficiencies, it is necessary to resonate the drive radiation within each optical cavity containing a NLO crystal, to build up the drive intensity within the cavity. To stabilize the intra-cavity intensities, the length of each coupled optical cavity must be controlled to a fraction of its drive wavelength. The drive wavelength and the optical cavity length fluctuate due to thermal and to mechanical motion effects in the environment, so these effects must be monitored and actively counteracted using feedback mechanisms. While a single stage of frequency conversion can be achieved stably and efficiently, the stability requirements for a second stage are extremely challenging. While this type of CW DUV laser is more than an order of magnitude more efficient and powerful than the earlier prior art based on the argon-ion gas laser, the requirement of controlling the lengths of the coupled optical cavities to a fraction of the drive wavelengths greatly increases the complexity and cost, and greatly lowers operational availability of this type of source. As with the prior art based on the argon ion gas laser, the solid state based DUV CW 266 nm laser source is subject to degradation of the NLO crystals being exposed to short DUV 266 nm wavelength, high intra-cavity intensity radiation. Thus the need persists for a practical, powerful (multi-watt), efficient (>1%) reliable CW DUV source, not subject to the 266 nm-caused degradation of a NLO crystal of the prior art.
To avoid the most deleterious aspects of the above-mentioned intrinsic limitations of NLO based approaches to CW DUV laser sources, several prior art methods have been described wherein the energies of several longer wavelength laser photons are combined, by various means described below, thereby creating DUV wavelength laser photons.
In discussing, these various means, it is important to differentiate several distinct types of multiple photon processes. Herein we use the following terminology:
(A) Sequential photon energy “summing,” or “summation” refers to the incoherent, time successive absorption of two or more photons from two or more drive pump lasers by a neutral atom, in which a valence electron is promoted from the ground electronic level successively to higher-lying real (non-virtual) electronic levels via on-resonance, parity-allowed, electric dipole transitions. The resulting electron population densities in the real electronic levels do not depend on any fixed relationships between the phases of electro-magnetic waves of the drive pump lasers.
(B) “Multi-photon-adding” refers to the coherent simultaneous absorption of two or more photons by a neutral atom, in which a valence electron is promoted from the ground electronic level to an excited electronic level of the same parity, via virtual, parity-allowed electric-dipole transitions. This multi-photon energy adding mechanism may occur in one of two types: 1) the involved virtual parity-allowed intermediate electronic levels lie in energy well away from any of the input photon energies; and 2) the input photon energies are detuned in energy from the resonance energy of a parity-allowed electric dipole transition between electronic levels of the atom being excited. The resulting electron population densities in real electronic levels depend on fixed phase relationships between the phases of electro-magnetic waves of the drive pump lasers.
In the related prior art shown in FIG. 1 Krupke [U.S. Pat. No. 6,693,942] disclosed a photon energy summing device in which the energies of two infrared wavelength photons are converted to the energy of a single visible wavelength photon in a gain mixture of alkali atoms and one or more buffer gases. The wavelength of radiation of one of the two drive pump lasers matches (is substantially equal to) the wavelength of either of the parity-allowed, electric-dipole D1 or D2 transitions of the alkali atoms. The wavelength of radiation of the second of the two drive pump lasers matches (is substantially equal to) the wavelength of one of a proscribed set of parity-allowed, electric dipole transitions whose initial electronic level energetically is the lowest 2P1/2 or 2P3/2 level of the alkali atom, and whose terminal level is the 2D3/2 level that is somewhat higher than the energies of the second lowest lying 2P3/2 and 2P1/2 levels. Collisions with buffer gas atoms or molecules relax electrons from the pumped 2D3/2 level to the second lowest 2P3/2 and 2P1/2 levels, generating a population inversion between the second lowest 2P1/2 electronic level and the 2S1/2 ground electronic level. This population inversion density enables laser emission generally in the visible spectral region. In practice this scheme suffers from two major deficiencies: 1) reliance on filling the upper laser level (the second lowest 2P1/2 level) via collisions with buffer gases is generally inefficient, because excited electrons may be transferred as well to lower lying 2S1/2, 2P3/2 and 2P1/2 levels; 2) population inversions are also created between the second lowest 2P3/2 and 2P1/2 levels and lower lying 2S1/2, 2D5/2 and 2D3/2 levels. These population inversions give rise to amplified spontaneous emission (ASE) on the parity-allowed electric dipole transitions generally with wavelengths in the infrared spectral region. If allowed to grow uncontrollably by ASE, these infrared transitions will deplete the energy of electrons in the second lowest 2P3/2 and 2P1/2 levels intended for the visible wavelength output beam, and possibly may even prevent achievement of a population inversion with respect to the 2S1/2 ground electronic level. To overcome these deficiencies of this prior art, some means must be conceived and implemented that better directs drive laser excitation preferentially into the upper laser electronic levels, and at means must be conceived and implemented to control and/or suppress deleterious ASE transitions. Additionally, based on two infrared drive photons, this prior art is limited to producing output photons whose wavelengths are generally in the visible spectral region, and not in the target DUV spectral region of the present application.
In related prior art shown in FIG. 2, Sulham and Perram (Applied Physics, B101, 57, 2010) have demonstrated the production of visible ASE emission upon irradiating neutral alkali atoms, or a mixture of an alkali atoms and one or more buffer gases, with two “drive” pump lasers neither of which is resonant, or matched, to electric dipole allowed transitions of the alkali atom. Rather, in this multi-photon-adding process, the two drive photons are simultaneously absorbed, exciting an electron from the 2S1/2 ground electronic level to higher lying level of the same parity (i.e., 2S1/2, 2D3/2 or 2D5/2 electronic level). In these so-called coherent two photon-pumped schemes, the output visible emission occurs on the parity-allowed electric dipole transition between the second lowest lying excited 2P1/2 level and the 2S1/2 ground electronic level.
In this laser scheme the Doppler peak cross-section of a purely 2-photon transition is proportional to the intensity of the drive laser. At the drive intensities of practical interest (˜10's of kW/cm2) this peak cross-section is generally orders of magnitude smaller than the typical peak Doppler cross-section of a parity-allowed electric dipole transition, (10−18 cm2 vs 10−11 cm2, respectively). Thus to achieve practical degrees of drive photon absorption in such 2-photon pumped devices, the operating atom density generally needs to be higher, (requiring higher operating temperatures) and the gain cell lengths need to be longer, than for devices based on successive 1-photon absorption parity allowed electric dipole transitions. Again, employing two infrared drive photons only, output wavelengths are limited to the visible spectral region (i.e., 400-450 nm) in this related prior art.
Extending the coherent multi-photon excitation process to more than 2 photons was disclosed in the prior art (FIG. 3), by Goldstone (U.S. Pat. No. 4,807,240), He proposed the conversion of the energies of several infrared photons to produce a single photon whose energy is greater than the energies of any of the input photons, via a coherent, non-resonant multi-photon-adding mechanism in alkali atoms, illustrated in FIG. 3. However, since its appearance in 1989, Goldstone's teaching has not led to any commercial realizations of such devices. Again using the simultaneous non-resonant absorption of multiple drive photons, effective absorption transitions cross-sections scale with drive photon beam intensities, and generally will be much smaller than the peak resonant cross-sections. Accordingly, the demand intensities in the Goldstone teachings are generally an order of magnitude higher than those of present invention. Moreover, monitoring and control of pump wavelength and their de-tuning off-sets from transition resonances is complex and costly, compared to on-resonance operation.